From the beginning of 21st century, automobile electronization and intelligentization have become the direction and front edge of the development of automobile industry technology, meanwhile the safety of automobile and driver-friendly features have also become a focus for many big automobile manufacturers. Automatic measurement system is not only a driver-friendly feature for drives, but also a major component of active safety system of automobiles. In rainy days or in environment with high humidity, heavy rainwater or fog on the surface of automobile windshield often affects the sight of drivers. However, existing technologies cannot provide an economical, practical, and mass market oriented method for sensing rain or fog on windshield automatically.
Traditionally, drivers would adjust the direction of automobile air outlet system manually, so that air could directly blow towards the windshield to disperse fog. However, frequently adjusting air outlet direction by hand will distract the driver and cause safety problems. Automatic demisting system has been put forward as a cutting-edge technology of electronic development of automobiles, which can automatically sense fogging degree on the inner surface of automobile windshield, and based on which, automatically adjust air outlet direction and volume of automobile air-conditioning system; and after the fog is blown away, it can automatically restore the air outlet direction and volume to its original state. To date, no application of automatic demisting system has been seen in automobile industry due to sensor issues, it is believed that automatic demisting system will gradually become a standard feature of automobiles in the near future. The key technology of automatic demisting system is fogging sensor technology, objectively speaking, up to now, no practical fogging sensor for automobiles has been found around the world.
Key technology of automatic wiper system in prior art is raindrop sensor technology, objectively speaking, up to now, raindrop sensor technology is still not mature enough around the world. Although some renowned automobile manufacturers, such as Volkswagen, Chrysler and Citroen have successfully equipped automatic wiper system on their cars, the immature raindrop sensor technology has brought a series of regrets including difficult installation, high cost and high mistaken action rate, etc.
Current raindrop sensor technologies can be divided into two types according to their installation forms, namely built-in type and built-out type; built-out type is to install a sensor on the outside of windshield, such as above engine hood, outside of automobile front, outer surface of automobile windshield, etc; the characteristic of built-out raindrop sensor is to directly contact and measure rainwater; as far as we know, built-out raindrop sensors can be divided into photoelectric type, conductive type, vibrating type and parallel-plate capacitor according to their work principle. Since most built-out raindrop sensors cannot be directly installed on outer surface of windshield within the scope where wiper can reach, they can only sense rainfall without sensing brushing effect of the wiper; consequently, only open-loop control system for wiper can be formed, and close-loop control system reflecting brushing effect of the wiper cannot be formed. In addition, built-out raindrop sensors have also such defects as easy to be polluted and damaged; hence, built-out raindrop sensor is seldom applied in automobile industry. Built-in raindrop sensor is usually installed on the inner surface of automobile windshield where the wiper can reach, which must adopt non-contact measurement technology, it has such advantages that it can sense the rainfall and brushing effect of the wiper simultaneous, so as to provide feedback signals for wiper control in time; it is installed inside of the vehicle which prevents being polluted or damaged from the environment. In modern automobile industry, built-in raindrop sensor has become the mainstream.
So far, most built-in raindrop sensors widely used in automobile industry are all photoelectric sensors. And the photoelectric sensors can be divided into two types; the first type is to install a camera on inner surface of automobile windshield, which utilizes imaging principle similar to camera to conduct image analysis and processing so as to sense variations of rainfall. The cost of apparatus, data processing circuit and computer for this kind of photoelectric sensor is very high, thus it has not been applied in automobile industry at present. So infra we only discuss the second type of photoelectric raindrop sensor which has been widely used in automobile industry. It is to install a pair of beam emitting and receiving devices on inner surface of automobile windshield, which utilize reflection and refraction of light, when raindrops are attached on outside surface of automobile windshield, the refractive index of light will change, thus resulting in change of receiving light intensity on the receiving end, then the raindrop sensor will transfer the change of light intensity into change of electric signals, and then sense the change of rainfall. Built-in photoelectric raindrop sensor has the following defects:
Small measurement area. When there is a light rain, the distribution of raindrops per unit area is not even, as a result, if the measurement area is small, the automatic wiper system will react slowly. In most cases, the emitter of photoelectric raindrop sensor is a point source of light, though lens or prism can be used to scatter the beam to a certain extent to expand refraction area, this will bring negative influence to the sensitivity of receiver component. Consequently, practical measurement area of photoelectric raindrop sensor is usually within 1–2 sq. centimeters; of course, more pairs of emitting and receiving ends can be used to enlarge measurement area, but this will greatly increase the cost.
Unable to measure thickness of rainwater adhered to windshield surface. When there is a heavy rain, outer surface of automobile windshield will be evenly covered by rainwater, in this case, the rainfall can only be measured by variations of thickness of rainwater adhered to the windshield surface, if it cannot measure the thickness variations of rainwater, the automatic wiper system will work slowly and then affect brushing effects. The working principle of photoelectric raindrop sensor is to use variations in refractive index of beams, however refractive index is only related to physical characteristics of the incidence plane, and it is irrespective of thickness of the substance. Therefore, photoelectric raindrop sensor cannot reflect variations in the thickness of rainwater.
Be vulnerable to interference of pollutants. When the outer surface of automobile windshield is polluted by pollutants (such as: dust, mud, stain and bodies of dead insects), the refractive index of beams will also change a lot, the automatic wiper will be caused undesired actions due to the fact that the photoelectric raindrop sensor cannot distinguish pollutants from rainwater.
High technical requirements for installation. Photoelectric raindrop sensor requires that the emitting and receiving end should have accurate installation angles and distances; meanwhile, it also has high requirements for both location and angle of the assistant lens and prism system; furthermore, considering influence of automobile vibration, the installation must be tight and stable to prevent the accuracy and sensitivity of the sensor from being affected by deviation of the beams which caused by automobile vibrating.
Bad adaptability. The working principle of photoelectric raindrop sensor has a bad adaptability to different automobile windshields, materials of the windshield will influence refractive index and reflectivity, thickness of the windshield will influence installation angle and location of both emitting end and receiving end, so different photoelectric raindrop sensors must be designed for different automobile windshields.
High cost of material, installation and man-hour. The cost of materials of photoelectric emitting and receiving device, lens and prism system and fasteners which photoelectric raindrop sensor required are higher, installation and man-hour cost are also very high due to high requirements for installation techniques, these are the fundamental reasons why automatic wiper system only appeared in medium and high class vehicles at present time.
In addition to the photoelectric raindrop sensor which is mentioned above, there are also a few capacitance raindrop sensors are used in prior art. For instance:
Chinese patent number: 02147854.6, this patent provides a built-out parallel-plate capacitance rainwater sensor formed by a parallel connection of several pieces of capacitance plates, the outer shell of the sensor has an inlet and outlet, which conducts direct-contact measurement of rainwater. This sensor has typical demerits of built-out rainwater sensor, i.e. it cannot reflect brushing effect of the wiper and cannot form close-loop control system; it is vulnerable to be polluted and damaged and has a complicated structure. This sensor adopts a traditional parallel-plate capacitance measurement principle.
Patent document U.S. Pat. No. 4,805,070A, the name of which is “capacitance coupled humidity sensor”, its sense element is resistance network; it utilizes the parallel-plate capacitance principle to send variations of the resistance to signal generator and receiver by two unchangeable parallel-plate capacitance couplings. The essential of this sensor is traditional resistor-based humidity sensor; this technical solution only provides a non-contact connection method between the signal processing circuit and the sense element (resistor network) to avoid the problem of crossing the windshield to connect the leads. In the technical solution of above mentioned patent, the sense element is installed on outer surface of automobile windshield, which conducts direct contact measurement of rainwater, and when the sense element is installed on outer surface of automobile windshield, it has strict requirements for both materials and installation technology. For instance: it must be erosion-resisting, rub-proof, and pollution-preventing and should not influence normal actions of the wiper, etc. Meanwhile, the problem of electric connection of sense element must be resolved if performing contact measurement by installing sense element on outer surface of automobile windshield, so the technical solution of above-mentioned patent provides a very complicated non-contact connection method which forming a parallel-plate coupling capacitor between the crack of two layers of glasses and outer surface of automobile windshield, and the installation cost of this method must be high due to its over complicated technology.
The measurement principle of the technical solution provided by U.S. Pat. No. 5,668,478A is very similar to the technical solution provided by U.S. Pat. No. 4,805,070A, which is also a contact way to measure the variations of resistance and perform signal coupling by parallel-plate capacitance, it has the same defects as U.S. Pat. No. 4,805,070A.
Patent Document EP0333564A1, The electric rainwater detection device provided by this patent has utilized the principle of capacitance measurement, but the sensor provided by this patent is a typical built-out contact rainwater measurement sensor which requires an isolated cover layer with good water-proof performance. This sensor has typical defects of built-out rainwater sensor, which cannot reflect brushing effect of the wiper, and is incapable of forming close-loop control system; besides, it is vulnerable to be polluted and damaged with complicated structure.
Patent document DE3937605A1, The measurement principle of this sensor is the principle of traditional parallel-plate capacitance measurement, the technical solution of this patent published is the capacitive plate and the water layer can form a capacitor independently when a water layer emerges on the windshield. Obviously, this can only constitute two independent parallel-plate capacitors. The primary defect of said sensor is that it requires the capacitor plate must be installed between the two layers of glasses and the two functional films, which will no doubt limits applicable scope of this sensor, Because the windshields applied in automobile industry is available in both double layers and single layer at present. Besides, this sensor also has defects such as complicated structure, difficult installation, and trouble of wiring, etc.